Additional new type C RNA mammalian viruses were isolated which are typical of the endogenous baboon viral group: 12 primate viruses from Old World monkey tissues; FS-1 feline virus from the European wildcat (Felis sylvestris); Mus caroli xenotropic murine virus. Also, three new isolates were obtained which are typical of a group of mammalian type C RNA viruses infectious for primates. These viruses (GBr-1, GBr-2, GBr-3) were isolated from the brains of nontumored gibbons. These viral isolates, as well as previously obtained primate isolates were characterized using biological, immunological and nucleic acid hybridization criteria. The origins of type C RNA viruses, their evolutionary relationships, and modes of transmission, were studied. These investigations yielded four distinct examples of type C virogene transfer between species remotely related phylogenetically. Type C viral proteins were found to bind preferentially to the RNA of the virus from which they were derived representing the first example of proteins with specific nucleic acid recognition properties. Selective breeding of mammalian strains with and without marker virogenes provided a new approach to the investigation of the evolution of multiple gene systems.